There are a number of different techniques for detecting objects in closed containers such as suitcases and boxes carried by airplanes which involve conveying the suitcases past an X-ray scanner. In particular, contraband such as drugs or explosive materials are sought to be detected by discerning their densities and/or atomic number using dual energy approaches. The dual energy atomic number approach relies on the fact that when an X-ray beam strikes material the energy of the beam is diminished either because of absorption (the photoelectric effect .mu..sub.pe) or because of scattering (the Compton scattering effect .mu..sub.cs) and that the probability of the photoelectric effect, .mu..sub.pe, changes markedly with increased energy while the compton scattering, .mu..sub.cs, does not. Since .mu..sub.pe is a function of atomic number/energy and .mu..sub.es is a function of atomic mumber, these expressions can be solved for atomic number by using two different X-ray energy levels, e.g., 40 Kev and 90 Kev. In the case of explosives the materials sought are organic, containing carbon, nitrogen and oxygen, and have an atomic number of around 7. Heavier metals such as iron and chromium often found in luggage have atomic numbers of 28 or higher, and aluminum and chlorine have atomic numbers of around 12. Therefore, there is a comfortable margin for detection of the organic explosives. See "Device and Method for Inspection of Baggage and Other Objects", Krug et al., U.S. Pat. No. 5,319,547.
Density is also used to detect explosives because they typically have a density of 1.2-1.9 gm/cm.sup.3 for military and 1-1.4 gm/cm.sup.3 for commercial grade explosives which are well separated from the densities of other materials commonly found in luggage. Since a single dimension X-ray system can only produce a two dimensional or areal density, that is, weight per unit area related to the projected area of an object, it is not entirely reliable: the projected density is a composite of all densities in the line of the X-ray beam and one material can mask another. To overcome this and other shortcomings a three-dimensional scanner was developed. See "Three-Dimensional Reconstruction Based on a Limited Number of X-Ray Projections", Bjorkholm et al., U.S. Pat. No. 5,442,672.
But even this approach is subject to failure when thin sheets of explosive or other contraband are imaged perpendicularly or transversely relative to the sheet. A sheet imaged on edge, i.e., aligned with a scanning beam, is highly contrasted and detectable but when it is crosswise or wholly perpendicular to the scanning beam its thin dimension gives a very low areal density, e.g., less than 1 gm/cm, easily obscured when combined with the other objects in the line of sight. Such sheets of material are most likely to be disposed or secreted in the broad sides of a suitcase, not in the narrower ends or top and bottom, so they are not likely to be seen on edge. The only present technique for detecting these sheets with good reliability are computerized axial tomography systems which are large, complex and expensive.